Hold-pressure control in multi-parting injection molding system
This is a continuation of co-pending application Ser. No. 07/070,850 filed on July 8, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally pertains to injection molding and is particularly directed to control of hold pressure in a multi-parting injection molding system. 
     Hold pressure is the pressure applied to plastic material injected into a cooling cavity after the cooling cavity is initially filled by an injection unit. Hold pressure is applied to force additional plastic material into the cooling cavity as the initially injected material shrinks in the mold upon cooling in order to assure that the plastic material completely fills the cavity. Otherwise the molded product may be crimped. 
     In the prior art, the injection unit is used to apply hold pressure. However, when using a multi-parting molding system, the use of the molding unit to apply hold pressure may increase the cycle time. Also, with typical stack mold systems, when the stack mold is opened and the injection unit is being recharged, the injection unit is separated from the remainder of the feed system that carries the plastic material to the cooling cavities, thereby reducing the time available for the injection unit to apply pressure. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved system and method of controlling hold pressure to cooling cavities when cyclic injection molding a thermoplastic material by utilizing a combination of a multi-parting mold system and an injection unit which together encompasses a bifurcated feed system with a molten core, the feed system comprising a valve, a first branch for feeding a first cooling cavity encompassed by a first parting surface and a second branch for feeding a second cooling cavity encompassed by a second parting surface. 
     The present invention also provides an improved system and method of controlling hold pressure to cooling cavities when cyclic injection molding a thermoplastic material by utilizing a combination of a multi-parting mold system and an injection unit which together encompass a bifurcated feed system with a molten core, the feed system comprising a first branch with a first valve for feeding a first cooling cavity encompassed by a first parting surface and a second branch with a second valve for feeding a second cooling cavity encompassed by a second parting surface. 
     The features of the invention are described with reference to the description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates one preferred embodiment of the present invention. 
     FIG. 2 illustrates an alternative preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a preferred embodiment of a multi-part molding system for practising the present invention includes a center part 10 and end parts 12 and 14. The center part 10 and one end part 12 join at a first parting surface 16 and define a first cooling cavity 18. The center part 10 and the other end part 14 join at a second parting surface 20 and define a second cooling cavity 22. The center part 10 may be either a center platten of a molding machine or the center part of a stack mold. An injection unit 32 and the center part 10 define a feed system with a molten core. The feed system includes a runner orifice 24, a first branch 26 and a second branch 28. The first branch 26 terminates in a gate 27 to the first cooling cavity 18; and the second branch 28 terminates in a gate 29 to the second cooling cavity 22. A valve 30 controls the flow of plastic material through the runner orifice 24. An injection unit 32 is disposed to inject molten thermoplastic material into the runner orifice 24. The injection unit 32 is separated from the remainder of the feed system when not being pressurized to inject the plastic material into the runner orifice 24. A piston means 34 is connected to the feed system for controlling the pressure in the first and second branches 26, 28. The piston means 34 may be hydraulic, as shown, or may be electrically or spring powered. 
     The method of controlling the hold pressure according to the embodiment of the invention illustrated in FIG. 1 includes the following steps: 
     (a) opening the valve 30; 
     (b) filling the first cooling cavity 18 by pressurizing the injection unit 32; 
     (c) shutting the valve 30 to withhold thermoplastic material 36 in the first cooling cavity 18; 
     (d) hold pressurizing the thermoplastic material 36 withheld by the valve 30 in the first cooling cavity 18; 
     (e) opening the valve 30; 
     (f) filling the second cooling cavity 22 by pressurizing the injection unit 32; 
     (g) shutting the valve 30 to withhold thermoplastic material 36 in the second cooling cavity 22; and 
     (h) hold pressurizing the thermoplastic material 36 withheld by the valve 30 in the second cooling cavity 22. 
     Hold pressure is applied to the thermoplastic material 36 withheld by valve 30 in the respective cooling cavities 18, 22 when the valve 30 is closed. 
     Step (d) may further include the step of 
     (i) further hold pressurizing the first cooling cavity 18 by protracting the piston means 34. 
     Step (h) further includes the step of 
     (j) further hold pressurizing the second cooling cavity 22 by protracting the piston means 34. 
     This method further includes the step of 
     (k) depressurizing the feed system between the valve 30 and the cooling cavities 18, 22 by retracting the piston means 34. 
     Referring to FIG. 2, an alternative preferred embodiment of a multi-part molding system for practising the present invention includes a center part 40 and end parts 42 and 44. The center part 40 and one end part 42 join at a first parting surface 46 and define a first cooling cavity 48. The center part 40 and the other end part 44 join at a second parting surface 50 and define a second cooling cavity 52. The center part 40 may be either a center platten of a molding machine or the center part of a stack mold. An injection unit 62 and the center part 40 define a feed system with a molten core. The feed system includes a runner orifice 54, and is bifurcated at a bifurcation junction 55 into a first branch 56 and a second branch 58. Alternatively, a bifurcation junction may be located in the injection unit, in which case, the injection unit and the center part each has two separate orifices, one for each branch of the feed system. In the system shown in FIG. 2, the first branch 56 terminates in a gate 57 to the first cooling cavity 48; and the second branch 58 terminates in a gate 59 to the second cooling cavity 52. A first valve 60 controls the flow of plastic material through the first branch 56. A second valve 61 controls the flow of plastic material through the second branch 58. An injection unit 62 is disposed to inject molten thermoplastic material into the runner orifice 54. The injection unit 62 is separated from the remainder of the feed system when not being pressurized to inject the plastic material into the runner orifice 54. A first piston means 64 is connected to the first branch 56 for controlling the pressure in the first branch 56. A second piston means 65 is connected to the second branch 58 for controlling the pressure in the second branch 58. The piston means 64, 65 may be hydraulic, as shown, or may be electrically or spring powered. 
     The method of controlling the hold pressure according to the embodiment of the invention illustrated in FIG. 2 includes the following steps: 
     (a) opening the first valve 60; 
     (b) filling the first cooling cavity 48 by pressurizing the injection unit 62; 
     (c) shutting the first valve 60 to withhold thermoplastic material in the first cooling cavity 48; 
     (d) hold pressurizing the thermoplastic material 66 withheld by the first valve 60 in the first cooling cavity 48; 
     (e) opening the second valve 61; 
     (f) filling the second cooling cavity 52 by pressurizing the injection unit 62; 
     (g) shutting the second valve 61 to withhold thermoplastic material in the second cooling cavity 52; and 
     (h) hold pressurizing the thermoplastic material 67 withheld by the second valve 61 in the second cooling cavity 52. 
     Hold pressure is applied to the thermoplastic material 66 withheld by valve 60 in the first cooling cavity 48 when the valve 60 is closed. 
     Hold pressure is applied to the thermoplastic material 67 withheld by valve 61 in the second cooling cavity 52 when the valve 61 is closed. 
     Step (d) may further include the step of 
     (i) further hold pressurizing the first cooling cavity 48 by protracting the first piston means 64. 
     Step (h) may further include the step of 
     (j) further hold pressurizing the second cooling cavity 52 by protracting the second piston means 65. 
     This method further includes the steps of 
     (k) depressurizing the first branch 56 between the first valve 60 and the first cooling cavity 48 by retracting the first piston means 64; and 
     (l) depressurizing the second branch 58 between the second valve 61 and the second cooling cavity 52 by retracting the second piston means 65. 
     It is usually desirable to obtain a high injection pressure at the gate to the cooling cavity immediately as the injection is started. But due to the feed system of a multi-parting injection molding system tending to be rather long in distance, and the hot molten plastic material quite compressible, such high injection pressure at the gate is difficult to obtain. It should be noted that the piston means of the system and method of the present invention may be used to increase initial pressure at the gate at the beginning of injection.